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Dark GRBs illuminate early star formation

BY KEITH COOPER

ASTRONOMY NOW

Posted: 8 June, 2009

Mysterious gamma-ray bursts (GRBs) that leave no optical afterglow are exploding in very dusty patches hiding plentiful star formation in galaxies whose light has taken 12.9 billion years to reach us. This is the conclusion of a new study of 14 of these ‘dark’ GRBs presented today at the 214th American Astronomical Society Meeting in Pasadena, California.

These gamma-ray bursts are thought to be the mighty explosions of the most massive stars in the Universe, which emit a narrow beam of highly energetic gamma rays. We see the GRB if the beam is pointed towards us. In comparison, the afterglow of the explosion is much fainter at other wavelengths. It took until 1997 before we were even able to spot a GRB afterglow.

Artwork representing the twin beams of a gamma-ray burst exploding in a dusty region of a distant galaxy. Image: Aurore Simonnet/Sonama State University/NASA Education and Public Outreach.

With spacecraft like NASA’s Swift, which can be on the scene of a GRB in a matter of seconds, locating the afterglows of GRBs has since become routine. However, a small population of GRBs steadfastly refuse to make their afterglow visible. Now astronomers think they know why.

One scenario that had been suggested was that the GRBs were so distant that the light of their afterglows had been redshifted beyond the range of optical telescopes. However, a team led by Daniel Perley and Joshua Bloom of the University of California at Berkeley trained one of the ten-metre W M Keck telescopes in Hawaii on the position of 14 dark GRBs, and found that they could see the host galaxies of eleven of them. The galaxies of the remaining three could not be seen, but each of them had some afterglow light visible, but hugely diminished compared to the norm. This disproved the idea that they were at distance greater than a light-travel time of 13 billion years (just 700,000 years after the big bang), as the light was not redshifted beyond our reach.

The eleven dark GRBs for which the Keck Telescope identified a host galaxy, which appear as smudges of light at these great distances. The circles indicate the location of the GRB. Image: Daniel Perley, Joshua Bloom/UC Berkeley.

Instead, Perley and Bloom’s team believe the reason we cannot see the afterglows is that they are shrouded in thick dust produced by rapid star formation and death, which blocks the light of the afterglow. Yet the host galaxies that they observed do not seem especially dusty. “The dust is probably in clouds and knots around the forming stars,” says Perley.

The consequence of this is that dark GRBs could be an excellent way of keeping tabs on star formation in galaxies so distant that we cannot otherwise detect star-forming regions. In turn, this could indicate the rate of star formation in these galaxies, and how fast these galaxies are growing.

Already, the lack of dark GRBs more than a light-travel distance of 12.9 billion years suggests that star-formation progressed at a more relaxed pace before that. “Gamma-ray bursts are so powerful that if these were frequent occurrences 13 billion years ago, we ought to be detecting large numbers of those same explosions today as high redshift events,” says team member S Bradley Cenko. “We don’t, which indicates that the first stars formed at a less frenzied pace than some models suggested.”

To that end, the team estimate that no more than seven percent of GRBs originate from more than 13 billion years ago.

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